However, when I run the Task associated with my PowerPoint Report, the Page feature is only applied to the Slides Header and not to the Chart inside it, i.e., the Fields selections are not applied. So i get multiple slides with different Headers but always the same chart.

Do you have triggers or alternate states in your QVW. Alternate states and triggers are not supported in NPrinting. NPrinting can only report on the default state of the document and triggers can cause unpredictable report results.

Check these for now and if you continue to experience issues, I suggest generating a ticket with the Qlik Support Desk to continue the troubleshooting effort if needed.